The invention relates to an optical beam shaper having an entrance surface and an exit surface located opposite thereto for changing the shape of the cross-section of a first radiation beam incident on the entrance surface, the entrance surface and the exit surface having a common optical axis. The invention also relates to a radiation source unit and to an optical scanning device, both including such a beam shaper.
A beam shaper of this type is known, for example in the form of a prism, a cylindrical lens or, as described in European Patent Application no. 0 286 368, a single lens element whose entrance and exit surfaces have a toroidal shape. This beam shaper is generally used in combination with a diode laser which emits a beam whose angular aperture in a plane parallel to its active layer, known as the lateral plane, is smaller than the angular aperture in the plane perpendicular to the active layer, known as the transversal plane. In the field known as the far field, the beam of such a diode laser has an elliptical cross-section. In a device in which such a diode laser is used as a radiation source, for example, a reading and/or writing device for optical record carriers in which an audio or video program or data are or can be stored, or a printer, a round and small, preferably diffraction-limited radiation spot must be formed on the medium to be scanned. To this end an imaging system, or objective system, by means of which the radiation spot is formed must be filled with a radiation beam having a circular cross-section. It is known that, starting from a diode laser, such a beam can be obtained by arranging a beam shaper between this laser and the objective system and at some distance from the diode laser.
In known systems using a beam shaper, stringent requirements must be imposed on the beam shaper as well as on the positioning of this element with respect to the radiation source. The known beam shapers are designed in such a way that the beam shaping, hence the magnification or reduction of the beam cross-section, is realised in only one of the planes, the transversal or the lateral plane. Since the shaping in this plane must be relatively strong, stringent requirements are imposed on the parameters playing a role in beam shaping.
Moreover, in known systems using beam shaping, the beam shaper is arranged at a relatively large distance from the radiation source, viz. where the diverging beam emitted by the source has the required cross-section. However, stringent requirements are imposed on the axial or Z position of the exit plane of the diode laser with respect to the beam shaper. If the Z position of the diode laser exit plane differs from the desired position, the laser beam will have a wavefront with a quadratic defocusing term at the area of the entrance surface of the beam shaper. The quadratic distortion which is a function of the angle at which a given portion of the wavefront is viewed from the centre of the radiation source is transformed in different manners by the beam shaper in the two main cross-sectional planes, the XZ plane and the YZ plane. In fact, the known beam shapers have a relatively large angular magnification factor or scaling factor in one of these planes and a magnification equal to one in the other plane. If the beam-shaping ratio is larger than, for example two, the defocusing of the radiation source is substantially completely transformed by the beam shaper into a defocusing of the beam in only one of the main cross-sectional planes. This means that the beam emerging from the beam shaper has become astigmatic. Whereas in the optical systems under consideration a defocusing of the radiation source itself can be corrected by an active focus control for the objective system, an astigmatic wavefront error can no longer be eliminated. Consequently, stringent tolerance requirements are imposed on astigmatism. If an average wavefront deviation, i.e. the square root of the integral, across the surface of the wavefront, of the square value of the wavefront deviation divided by the surface, indicated by OPD.sub.rms, of 0.02.times.the wavelength (.lambda.) is still allowed, the astigmatic wavefront error W.sub.A should be smaller than 0.1 .lambda.. This means that the defocusing .DELTA.Z of the radiation source with respect to the beam shaper defined by ##EQU1## may at most be of the order of 1.5 .mu.m if the numerical aperture NA of the beam shaper is 0.35 and .lambda.=0.8 .mu.m.
In the commonly used optical systems, in which the beam shaper is arranged at a relatively large distance from the diode laser, such a strict tolerance requirement is difficult to satisfy. Due to temperature variations and mechanical shocks, axial displacements amounting to many microns may occur between the diode laser and the beam shaper.